Iron castings with compacted or spheroidal graphite produced by determining coefficients from cooling curves and adjusting the content of structure modifying agents in the melt

ABSTRACT

A sampling device for thermal analysis of solidifying metal, comprising at least one container intended to contain a sample quantity ( 30 ) of liquid metal during analysis, and at least one sensor ( 40, 220, 240 ) for thermal analysis, said sensor(s) being intended to be at least partly immersed in the solidifying metal sample quantity during analysis. The container comprises an inner wall ( 50 ), with an interior surface ( 60 ) intended to face the sample quantity during analysis, and an exterior surface ( 70 ); an outer wall ( 80 ), with an exterior surface ( 100 ) intended to face the ambient atmosphere, and an interior surface ( 90 ); said walls being joined at the mouth of the container whereby the exterior surface ( 70 ) of the inner wall ( 50 ) and the interior surface ( 90 ) of the outer wall ( 80 ) together define an essentially closed space ( 110 ).

This application is the national phase of international applicationPCT/SE98/02072 filed Nov. 17, 1998 which designated the U.S.

The present invention relates to an improved method for predicting themicrostructure with which a certain cast iron melt will solidify. Theinvention also relates to an apparatus for carrying out the method.

BACKGROUND OF THE INVENTION

WO86/01755 (incorporated by reference) discloses a method for producingcompacted graphite cast iron by using thermal analysis. A sample istaken from a bath of molten cast iron and this sample is permitted tosolidify during 0.5 to 10 minutes. The temperature is recordedsimultaneously by two temperature responsive means, one of which isplaced in the centre of the sample and the other in the immediatevicinity of the vessel wall. So-called cooling curves representingtemperature of the iron sample as a function of time are recorded foreach of the two temperature responsive means. According to this documentit is then possible to determine the necessary amount ofstructure-modifying agents that must be added to the melt in order toobtain the desired microstructure. However, no detailed information isgiven about how to evaluate the curves.

WO92/06809 (incorporated by reference) describes a specific method forevaluating the cooling curves obtained by the method of WO86/01755.According to this document, an early plateau in the cooling curveindicates that flake graphite crystals have precipitated close to thetemperature responsive means. As the sample vessel is intentionallycoated with a layer of oxide or sulfide-bearing material which consumesthe active form of the structure-modifying agents, and thus simulatesits natural loss or fading during the casting period, such a plateau canoften be found in a cooling curve from a temperature responsive meansarranged close to the vessel wall. The skilled person can then determinewhether any structure-modifying agent has to be added to the melt inorder to obtain compacted graphite cast iron by using calibration data.

The method of WO92/06809 requires “perfect” curves comprising a distinctplateau. However, sometimes cooling curves without a distinct plateauare recorded despite the fact that flaky graphite has been formed. Up tonow, it has not been possible to use curves without a distinct plateauas a basis for calculating the precise amount of structure-modifyingagent that must to be added to the melt in order to produce compactedgraphite cast iron over the entire casting period.

SUMMARY OF THE INVENTION

Now, it has turned out that it is possible to use virtually any set ofcooling curves obtained for eutectic and under-eutectic solidification,and by the equipment of WO86/01755 and WO92/06809 as a basis forcalculating the precise amount of structure-modifying agent that must beadded. The method of the present invention comprises the steps of:

a) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron, orspheroidal graphite cast iron, as a function of γ, where

γ=(TA _(max) −TA _(min))/(TB _(max) −TB _(min))

 and wherein

TA_(max) is the local maximum value of the cooling curve recorded at thecentre of the sample vessel;

TA_(min) is the local minimum value of the cooling curve recorded at thecentre of the sample vessel;

TB_(max) is the local maximum value of the cooling curve recorded at thesample vessel wall;

TB_(min) is the local minimum value of the cooling curve recorded at thesample vessel wall;

b) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron, orspheroidal graphite cast iron, as a function of φ, where

φ=(TA′ _(max)/(TB′ _(max))

 wherein

TA′_(max) is the maximum value of the first derivative of the coolingcurve recorded at the centre of the sample vessel; and

TB′_(max) is the maximum value of the first derivative of the coolingcurve recorded at the sample vessel wall;

c) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron, orspheroidal graphite cast iron as a function of the area (ρ_(B)) of thefirst peak of the first derivative of the cooling curve recorded at thesample vessel wall;

d) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron, orspheroidal graphite cast iron as a function of κ, where:

κ=σ_(A)/σ_(B)

 wherein

σ_(A) is the area under the second peak of the first derivative of thecooling curve recorded in the centre of the sample vessel; and

σ_(B) is the area under the second peak of the first derivative of thecooling curve recorded at the vessel wall;

e) recording cooling curves at the centre of the sample vessel and atthe sample vessel wall, respectively, for a particular sample of amolten cast iron;

f) depending on the result in e) choosing one of the calibration curvesfrom step a)-d) giving the most accurate result; and

g) calculating the amount of structure modifying agent that has to beadded to the melt.

DETAILED DESCRIPTION OF THE INVENTION

As already mentioned, the present invention relates to an improvedmethod for predicting the microstructure in which a certain cast ironmelt will solidify. By using the present method, it is possible toevaluate a much larger range of temperature time curves compared to thestate of the art and it is also possible to obtain more accurateresults.

The term “cooling curve” as utilized herein refers to graphsrepresenting the temperature as a function of time, which graphs havebeen recorded in the manner disclosed in WO86/01755 and WO92/06809.

The term “sample vessel” as disclosed herein, refers to a small samplecontainer which, when used for thermal analysis, is filled with a sampleof molten metal. The temperature of the molten metal is then recordedduring solidification in a suitable way. The walls of the sample vesselare coated with a material which reduces the amount ofstructure-modifying agent in the melt in the immediate vicinity of thevessel wall. Preferably the sample vessel is designed in the mannerdisclosed in WO86/01755, WO92/06809, WO91/13176 (incorporated byreference) and WO96/23206 (incorporated by reference).

The term “sampling device” as disclosed herein, refers to a devicecomprising a sample vessel equipped with at least one temperatureresponsive means for thermal analysis, said means being intended to beimmersed in the solidifying metal sample during analysis, and a meansfor filling the sample vessel with molten metal. The sample vessel ispreferably equipped with said sensors in the manner disclosed inWO96/23206.

The term “structure-modifying agent” as disclosed herein, relates tocompounds either promoting spheroidization or precipitation of thegraphite present in the molten cast iron. Suitable compounds can bechosen from the group of inoculating substances well-known in the art,and shape-modifying agents, such as magnesium, cerium and other rareearth metals. The relationship between the concentration ofstructure-modifying agents in molten cast irons and the graphitemorphology of solidified cast irons have already been discussed in theabove cited documents WO92/06809 and WO86/01755.

The invention also relates to an apparatus for controlling theproduction of compacted graphite cast iron, which apparatus takes asample of molten cast iron, uses the present method for calculating thenecessary additions, if any, of structure-modifying agents to the moltencast iron, and provides the molten cast iron with said amount ofstructure-modifying agents. The apparatus comprises a sampling device, acomputer-based data acquisition system, and a means for administratingstructure-modifying agents to the molten cast ron. The sampling devicecontains a representative sample of the molten cast iron which issubjected to thermal analysis during which temperature/time measurementsare transmitted to a computer and presented in the form of coolingcurves. The computer calculates the necessary amount ofstructure-modifying agent that must be added and automatically actuatesthe means for administrating the structure-modifying agent, whereby themelt is supplied with an appropriate amount of such agents.

The invention will now be described with reference to the accompanyingfigures in which:

FIG. 1 is a cross section through a part of a sampling device that canbe used in connection with the present invention;

FIG. 2 discloses examples of cooling curves recorded with twotemperature responsive means, one being arranged in the middle of thesample vessel (curve I) and the other near the vessel wall (curve II);

FIG. 3 shows a cooling curve corresponding to curve II in FIG. 2. Thefirst time derivative of the curve is also disclosed;

FIG. 4A defines the parameters TB′_(max), TB_(max) TB_(min). The figureshows TB values and σ_(B) for the part of a wall-region cooling curvecomprising a wall-region conventional undercooling recalescence andsteady state growth. The centre curve parameters are generally markedwith a capital A whereas wall parameters are marked with a capital B.

FIG. 4B shows three different appearances of the curve depending on theamount of flake graphite growth during the initial stages ofsolidification;

FIG. 5 demonstrates currents in a sample of solidifying molten metal andhow these currents affect the layer of flake graphite cast iron normallyformed in the vicinity of the vessel wall;

FIG. 6 is a schematic presentation of an apparatus for controllingproduction of compacted graphite cast iron according to the presentinvention.

As mentioned above, FIG. 1 shows the metal-containing part of a samplingdevice 200 that can be used when carrying out the present method. Meansfor filling a sample of molten metal into a sample vessel is not shown.Device 200 is equipped with two sensors, arranged essentially inaccordance with the teachings of WO86/01755 cited above. The temperaturesensing part 210 of the first temperature responsive sensor 220 isplaced in the centre of the molten metal 30, and the temperature-sensingpart 230 of the second sensor 240 is arranged at a location close to theinterior surface 60 (which may or may not be coated; coating not shown)of the inner wall 50. A sensor support member 250 is provided to holdthe sensors 220, 240 in position during analysis. The sensor supportmember is connected to the container by legs 255, between which moltenmetal flows into the container when immersed.

FIG. 2 shows an example of a set of cooling curves recorded form twotemperature responsive means, one being arranged in the middle of thesample vessel (curve I) and the other near the vessel wall (curve II).Curve I is a typical curve for the compacted graphite solidification inthe centre of the sample. The first inflexion point, or thermal arrest,is caused by the formation of primary austenite which is common forhypoeutectic cast irons. In contrast, the inflexion point in curve IIindicates the local formation of flake graphite caused by aninsufficiency of structure-modifying agent after reaction with the wallcoating. Curve II and its corresponding first time derivative is alsodisclosed in FIG. 3. In this case there is a relationship between thearea of the first peak (ρ_(B)) of the first time derivative of thecooling curve and the amount of flake graphite formation in the vicinityof the vessel wall.

When a casting/probe solidifies in a mould/sample vessel, any oxygen,sulphur, etc. in the atmosphere or in the mould/sample vessel materialmay react with the structure-modifying agents in the cast iron. Forcompacted graphite cast iron this may result in the formation of flakegraphite near the wall of the mould/sample vessel. In fact the amount offlake graphite formed is larger when the concentration ofstructure-modifying agents is lowered. Hence, the amount of flakegraphite formed at the wall can be used as a measure of the concentationof residual structure-modifying agents in the bulk of the metal.

Because flake graphite is nucleated at a higher undercooling temperaturethan compacted graphite, it can be distinguished by thermal analysis.FIG. 3 shows a cooling curve and corresponding first derivative recordedclose to the wall where both flake graphite and compacted graphite areformed. The amount of flake graphite formation can be monitored bymeasuring the area ρ_(B) of the first peak of the first derivative ofthe temperature time curve. The amount of compacted graphite formationcan analogously be monitored by measuring the area σ_(B) of the secondpeak of the first derivative of the temperature time curve.

However, because of the shape of the cooling curve it is sometimes notpossible to calculate either or both of the above defined areas ρ and σ.Examples of curves recorded near the wall which are diverging from theideal curve shape (curve II in FIG. 2 and FIG. 3) are given in FIG. 4B.Until now it has not been possible to evaluate results as represented bycurves T_(B1), T_(B2) and T_(B3), and in cases where such curves wereobtained the measurement had to be repeated, resulting in productivitylosses and possibly rejected iron due to excessive temperature loss.

According to the present invention, an analysis of the cooling curvescan be based upon the following fact: As the amount of flake graphiteformation increases, the amount of compacted graphite formation mustdecrease since the total amount of released carbon is approximatelyconstant. FIG. 4A shows a cooling curve recorded near the wall relatingto a case where only compacted graphite is formed. The formation ofcompacted graphite is characterized by the positive maximal slope of thecurve (T′_(Bmax)) the recalescence (T_(Bmax)−T_(Bmin)) and the areaσ_(B). FIG. 4B displays the same curve with progressively increasingamounts of flake graphite formation. Both the recalescence, the maximalslope, and the area under the T′_(B) peak decrease as the amount offlake graphite increases.

The amount of heat liberated by the initial formation of flake graphitein the near-wall region is very small, and indeed insufficient to berelied upon as control parameter. However, if the shape of the bottom ofthe sample vessel is predominantly spheroidal; and, if the vessel itselfis preheated (for example by immersion into the molten iron) thusavoiding formation of a chill zone of solidified iron in the near-wallregion; and, if the vessel is allowed to hang freely so that heat is notextracted into a floor or mounting stand, a favourable convectioncurrent will develop within the molten iron contained in the samplevessel. These convection currents “rinse” the flake graphite away fromthe pre-heated upper walls of the sample vessel and effectivelyconcentrate the flake growth in a flow-separated region at the base ofthe essentially spheroidal vessel. By strategically positioning thewall-sensor within the flow-separated area, one obtains a larger andmore sensitive measurement of the flake graphite wall reaction.

The method of the present invention requires four calibrations in orderto be carried out, namely:

a) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron, orspheroidal graphite cast iron, as a function of γ, where

γ=(TA _(max) −TA _(min))/(TB _(max) −TB _(min))

 and wherein

TA_(max) is the local maximum value of the cooling curve recorded at thecentre of the sample vessel;

TA_(min) is the local minimum value of the cooling curve recorded at thecentre of the sample vessel;

TB_(max) is the local maximum value of the cooling curve recorded at thesample vessel wall;

TB_(min) is the local minimum value of the cooling curve recorded at thesample vessel wall;

b) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron, orspheroidal graphite cast iron, as a function of φ, where

φ=(TA′ _(max))/(TB′ _(max))

 wherein

TA′_(max) is the maximum value of the first derivative of the coolingcurve recorded at the centre of the sample vessel; and

TB′_(max) is the maximum value of the first derivative of the coolingcurve recorded at the sample vessel wall;

c) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron as afunction of the area (ρ_(B)) of the first peak of the first derivativeof the cooling curve recorded at the sample vessel wall;

d) determining the amount of structure-modifying agent that has to beadded to the melt in order to obtain compacted graphite cast iron, orspheroidal cast iron as a function of κ, where:

κ=σ_(A)/σ_(B)

 wherein

σ_(A) is the area under the second peak of the first derivative of thecooling curve recorded in the centre of the sample vessel; and

σ_(B) is the area under the second peak of the first derivative of thecooling curve recorded at the vessel wall;

Naturally, corresponding calibrations are carried out when producingspheroidal graphite cast iron.

Most of the calibrations are based on cooling curves recorded in thecentre of the sample vessel. The reason for this is that normally thereis no flake formation in the centre and hence, TA_(max)−TA_(min),TA′_(max) and σ_(A) are not negatively affected by flake graphiteprecipitation. The centre can accordingly be used as a reference pointeven when modification is so low that flake graphite is formed at thewall.

The amount of structure-modifying agent that has to be added to aparticular sample is calculated after carrying out a conventionalthermal analysis as described in the previously cited documentsWO86/01755 and WO92/06809. The cooling curves are then analyseddetermining γ, φ, ρ_(B) and κ. Three independent determinations of theamount of structure-modifying agents that has to be added are carriedout, and it is then simple for the skilled person to choose thedetermination giving the most accurate result.

It is preferred to carry out the prediction method by using acomputer-controlled system, especially when a large number ofmeasurements must be carried out. In this case the same kind of samplingdevice 22 that has been described above is used. Such acomputer-controlled system is outlined in FIG. 6. During the measurementof a particular sample the two temperature responsive means 10, 12 sendsignals to a computer 14 comprising a ROM unit 16 and a RAM unit 15 inorder to generate the cooling curves. The computer has access to theabove mentioned calibration data in a ROM unit 16 and calculates theamount of structure-modifying agents that must be added to the melt.This amount is signalled to a means 18 for administratingstructure-modifying agent to the melt 20 to be corrected, whereby themelt is supplied with an appropriate amount of such agents.

What is claimed is:
 1. A process for producing a compacted graphite ironcasting, or spheroidal graphite cast iron, requiring a sampling device,means for monitoring temperature as a function of time and a means foradministrating structure-modifying agents to a molten cast iron fromwhich said casting is to be produced, said method comprising the stepsof: a) for the chosen casting method carrying out the followingcalibrations: i) determining the amount of structure-modifying agentthat has to be added to the melt in order to obtain compacted graphitecast iron, or spheroidal graphite cast iron, as a function of a firstcontrol coefficient γ, where κ=(TA _(max) −TA _(min))/(TB _(max) −TB_(min))  and wherein TA_(max) is the local maximum value of the coolingcurve recorded at the centre of the sample vessel during solidificationof a cast iron sample; TA_(min) is the local minimum value of thecooling curve recorded at the centre of the sample vessel duringsolidification of a cast iron sample; TB_(max) is the local maximumvalue of the cooling curve recorded at the sample vessel wall duringsolidification of a cast iron sample; TB_(min) is the local minimumvalue of the cooling curve recorded at the sample vessel wall during'solidification of a cast iron sample; ii) determining the amount ofstructure-modifying agent that has to be added to the melt in order toobtain compacted graphite cast iron, or spheroidal graphite cast iron,as a function of a second control coefficient φ, where φ=(TA′_(max))/(TB′ _(max))  wherein TA′_(max) is the maximum value of thefirst derivative of the cooling curve recorded at the centre of thesample vessel during solidification of a cast iron sample; and TB′_(max)is the maximum value of the first derivative of the cooling curverecorded at the sample vessel wall during solidification of a cast ironsample; iii) determining the amount of structure-modifying agent thathas to be added to a molten cast iron in order to obtain compactedgraphite cast iron, or spheroidal graphite cast iron, as a function of athird control coefficient (σ_(B)), which is the area under the firstpeak of the first derivative of a cooling curve recorded at the samplevessel wall during solidification of a cast iron sample; iv) determiningthe amount of structure-modifying agent that has to be added to the meltin order to obtain compacted graphite cast iron, or spheroidal cast ironas a function of a fourth control coefficient κ, where: κ=σ_(A)/σ_(B) wherein σ_(A) is the area under the second peak of the first derivativeof the cooling curve recorded in the centre of the sample vessel; andσ_(B) is the area under the second peak of the first derivative of thecooling curve recorded at the vessel wall; b) during solidificationrecording cooling curves at the centre of a sample vessel and at thesample vessel wall, respectively, for a particular sample of a moltencast iron; c) calculating control coefficients γ, φ, ρ_(B) and κrelating to the temperature time curves obtained in step b) and choosingone of these coefficients γ, φ, ρ_(B) and κ giving the most accurateresult; d) calculating the amount of stucture modifying agent (Va) thathas to be added to the melt; e) add the calculated amount of structuremodifying agent; and f) carry out the casting operation in a per seknown manner.
 2. A process according to claim 1, characterized in thatan essentially spheroidal sample vessel is used, and in that coolingcurves recorded near the vessel wall are recorded in a flow-separatedarea at the base of said essentially spheroidal sample vessel.
 3. Aprocess according to claim 1 or claim 2, characterized in that compactedgraphite cast iron is produced.
 4. A method for determining the amountof structure modifying agent that has to be added to molten cast iron inorder to produce a compacted graphite iron casting, or spheroidalgraphite cast iron, which method requires a sampling device, means formonitoring temperature as a function of time and a means foradministating structure-modifying agents to a molten cast iron fromwhich said casting is to be produced, said method comprising the stepsof: a) for the chosen casting method carrying out the followingcalibrations: i) determining the amount of structure-modifying agentthat has to be added to the melt in order to obtain compacted graphitecast iron, or spheroidal graphite cast iron, as a function of a firstcontrol coefficient γ, where γ=(TA _(max) −TA _(min))/(TB _(max) −TB_(min))  and wherein TA_(max) is the local maximum value of the coolingcurve recorded at the centre of the sample vessel during solidificationof a cast iron sample; TA_(min) is the local minimum value of thecooling curve recorded at the centre of the sample vessel duringsolidification of a cast iron sample; TB_(max) is the local maximumvalue of the cooling curve recorded at the sample vessel wall duringsolidification of a cast iron sample; TB_(min) is the local minimumvalue of the cooling curve recorded at the sample vessel wall duringsolidification of a cast iron sample; ii) determining the amount ofstructure-modifying agent that has to be added to the melt in order toobtain compacted graphite cast iron, or spheroidal graphite cast iron,as a function of a second control coefficient φ, where φ=(TA′_(max))/(TB′ _(max))  wherein TA′_(max) is the maximum value of thefirst derivative of the cooling curve recorded at the centre of thesample vessel during solidification of a cast iron sample; and TB′_(max)is the maximum value of the first derivative of the cooling curverecorded at the sample vessel wall during solidification of a cast ironsample; iii) determining the amount of structure-modifying agent thathas to be added to a molten cast iron in order to obtain compactedgraphite cast iron, or spheroidal graphite cast iron, as a function of athird control coefficient (ρ_(B)), which is the area under the firstpeak of the first derivative of a cooling curve recorded at the samplevessel wall during solidification of a cast iron sample; d) determiningthe amount of structure-modifying agent that has to be added to the meltin order to obtain compacted graphite cast iron, or spheroidal cast ironas a function of κ, where: κ=σ_(A)/σ_(B)  wherein σ_(A) is the areaunder the second peak of the first derivative of the cooling curverecorded in the centre of the sample vessel; and σ_(B) is the area underthe second peak of the first derivative of the cooling curve recorded atthe vessel wall; b) during solidification recording cooling curves atthe centre of a sample vessel and at the sample vessel wall,respectively, for a particular sample of a molten cast iron; c)calculating control coefficients γ, φ, ρ_(B) and κ relating to thetemperature time curves obtained in step b) and choosing one of thesecoefficients γ, φ, ρ and κ giving the most accurate result; d)calculating the amount of stuctural modifying agent (Va) that has to beadded to the melt.
 5. A process according to claim 4, characterized inthat an essentially spheroidal sample vessel is used, and in thatcooling curves recorded near the vessel wall are recorded in aflow-separated area at the base of said essentially spheroidal samplevessel.
 6. A method according to claim 4 or claim 5, characterized inthat a compacted graphite iron casting is produced.
 7. An apparatus forestablishing, in real time, an amount of a structure modifying agent tobe added to a cast iron melt (20) during the process of producing acompacted graphite iron casting; the apparatus comprising: a firsttemperature sensor (10) for recording a cooling curve at the centre of asample vessel; a second temperature sensor (12) for recording a coolingcurve in the vicinity of the sample vessel wall; a computer device (14)for determining an amount value (Va) of a structure modifying agent tobe added to the melt, a memory means (16) which is provided withprerecorded cooling curve data, the computer device being set up toestablish a first control coefficient, γ, (from which a first predictionvalue (V1) can be calculated,) where  γ=(TA _(max) −TA _(min))/(TB_(max) −TB _(min))  and wherein TA_(max) is the local maximum value ofthe cooling curve recorded at the centre of the sample vessel duringsolidification of a cast iron sample; TA_(min) is the local minimumvalue of the cooling curve recorded at the centre of the sample vesselduring solidification of a cast iron sample; TB_(max) is the localmaximum value of the cooling curve recorded at the sample vessel wallduring solidification of a cast iron sample; TB_(min) is the localminimum value of the cooling curve recorded at the sample vessel wallduring solidification of a cast iron sample; the computer device beingset up to establish a second control coefficient φ, (from which a secondprediction value (V2) can be calculated,) where φ=(TA′ _(max))/(TB′_(max))  wherein TA′_(max) is the maximum value of the first derivativeof the cooling curve recorded at the centre of the sample vessel duringsolidification of a cast iron sample; and TB′_(max) is the maximum valueof the first derivative of the cooling curve recorded at the samplevessel wall during solidification of a cast iron sample; the computerdevice being set up to attemt to establish a third control coefficient(ρ_(B)), (from which a third prediction value (V3) can be calculated,)where the third control coefficient (ρ_(B)) relates to the area of thefirst peak of the first derivative of the cooling curve recorded at thesample vessel wall; the computer device being set up to attemt toestablish a fourth control coefficient (κ), (from which a fourthprediction value (V4) can be calculated), where κ=σ_(A)/σ_(B)  andwherein σ_(A) is the area under the second peak of the first derivativeof the cooling curve recorded in the centre of the sample vessel; andσ_(B) is the area under the second peak of the first derivative of thecooling curve recorded at the vessel wall; the computer device being setup to compare the first, second, third and fourth control coefficients(γ, φ, σ_(B) and κ) with the prerecorded cooling curve data, and thecomputer device being set up to choose one of the control coefficients(γ, φ, σ_(B) and κ) in response to the result of the comparison, andwherein the computer device is set up to calculate a precise amountvalue (Va) of a structure modifying agent to be added to the melt inresponse to the choosen control coefficient (γ, φ, ρ_(B) and κ).
 8. Anapparatus according to claim 7, characterized in that the secondtemperature sensor (12) is arranged in such a way that the coolingcurves recorded near the wall of the sample vessel wall are recorded ina flow-separated area at the base of an essentially spheroidal samplevessel.
 9. An apparatus for establishing, in real time, an amount of astructure modifying agent to be added to a cast iron melt (20) duringthe process of producing a speroidal graphite iron casting; theapparatus comprising a first temperature sensor (10) for recording acooling curve at the centre of a sample vessel; a second temperaturesensor (12) for recording a cooling curve in the vicinity of the samplevessel wall; a computer device (14) for determining an amount value (Va)of a structure modifying agent to be added to the melt, a memory means(16) which is provided with prerecorded cooling curve data, the computerdevice being set up to establish a first control coefficient, γ, (fromwhich a first prediction value (V1) can be calculated,) where γ=(TA_(max) −TA _(min))/(TB _(max) −TB _(min))  and wherein TA_(max) is thelocal maximum value of the cooling curve recorded at the centre of thesample vessel during solidification of a cast iron sample; TA_(min) isthe local minimum value of the cooling curve recorded at the centre ofthe sample vessel during solidification of a cast iron sample; TB_(max)is the local maximum value of the cooling curve recorded at the samplevessel wall during solidification of a cast iron sample; TB_(min) is thelocal minimum value of the cooling curve recorded at the sample vesselwall during solidification of a cast iron sample; the computer devicebeing set up to establish a second control coefficient φ, (from which asecond prediction value (V2) can be calculated,) where  φ=(TA′_(max))/(TB′ _(max))  wherein TA′_(max) is the maximum value of thefirst derivative of the cooling curve recorded at the centre of thesample vessel during solidification of a cast iron sample; and TB′_(max)is the maximum value of the first derivative of the cooling curverecorded at the sample vessel wall during solidification of a cast ironsample; the computer device being set up to attemt to establish a thirdcontrol coefficient (ρ_(A)), (from which a third prediction value (V3)can be calculated,) where the third control coefficient (ρ_(B)) relatesto the area of the first peak of the first derivative of the coolingcurve recorded at the sample vessel wall; the computer device being setup to attemt to establish a fourth control coefficient (κ), (from whicha fourth prediction value (V4) can be calculated), where κ=σ_(A)/σ_(B) and wherein σ_(A) is the area under the second peak of the firstderivative of the cooling curve recorded in the centre of the samplevessel; and σ_(B) is the area under the second peak of the firstderivative of the cooling curve recorded at the vessel wall; thecomputer device being set up to compare the first, second, third andfourth control coefficients (γ, φ, ρ_(B) and κ) with the prerecordedcooling curve data, and the computer device being set up to choose oneof the control coefficients (γ, φ, ρ_(B) and κ) in response to theresult of the comparison, and wherein the computer device is set up tocalculate a precise amount value (Va) of a structure modifying agent tobe added to the melt in response to the choosen control coefficient (γ,φ, ρ_(B) and κ).
 10. An apparatus according to claim 9, characterized inthat the second temperature sensor (12) is arranged in such a way thatthe cooling curves recorded near the wall of the sample vessel wall arerecorded in a flow-separated area at the base of an essentiallyspheroidal sample vessel.
 11. An apparatus for carrying out the processof claims 1 or 2, the apparatus comprising: a sampling device (22) fortaking a sample of molten cast iron from a cast iron melt (20) fromwhich a casting comprising CGI or SGI is to be produced; a firsttemperature sensor (10) for recording a cooling curve at the centre of asample vessel; a second temperature sensor (12) for recording a coolingcurve in the vicinity of the sample vessel wall; a computer device (14)for determining an amount value (Va) of a structure modifying agent tobe added to the melt, a memory means (16) which is provided withprerecorded cooling curve data, a means (18) for administrating acorrect amount of a structure-modifying agent in response to a signalfrom the computer device, said signal corresponding to said amount value(Va) the computer device being set up to establish a first controlcoefficient, κ, (from which a first prediction value (V1) can becalculated,) where γ=(TA _(max) −TA _(min))/(TB _(max) −TB _(min))  andwherein TA_(max) is the local maximum value of the cooling curverecorded at the centre of the sample vessel during solidification of acast iron sample; TA_(min) is the local minimum value of the coolingcurve recorded at the centre of the sample vessel during solidificationof a cast iron sample; TB_(max) is the local maximum value of thecooling curve recorded at the sample vessel wall during solidificationof a cast iron sample; TB_(min) is the local minimum value of thecooling curve recorded at the sample vessel wall during solidificationof a cast iron sample; the computer device being set up to establish asecond control coefficient φ, (from which a second prediction value (V2)can be calculated,) where φ=(TA′ _(max))/(TB′ _(max))  wherein TA′_(max)is the maximum value of the first derivative of the cooling curverecorded at the centre of the sample vessel during solidification of acast iron sample; and TB′_(max) is the maximum value of the firstderivative of the cooling curve recorded at the sample vessel wallduring solidification of a cast iron sample; the computer device beingset up to attemt to establish a third control coefficient (ρ_(A)), (fromwhich a third prediction value (V3) can be calculated,) where the thirdcontrol coefficient (ρ_(B)) relates to the area of the first peak of thefirst derivative of the cooling curve recorded at the sample vesselwall; the computer device being set up to attemt to establish a fourthcontrol coefficient (κ), (from which a fourth prediction value (V4) canbe calculated), where κ=σ_(A)/σ_(B)  and wherein σ_(A) is the area underthe second peak of the first derivative of the cooling curve recorded inthe centre of the sample vessel; and σ_(B) is the area under the secondpeak of the first derivative of the cooling curve recorded at the vesselwall; the computer device being set up to compare the first, second,third and fourth control coefficients (γ, φ, ρ_(B) and κ) with theprerecorded cooling curve data, and the computer device being set up tochoose one of the control coefficients (γ, φ, ρ_(B) and κ) in responseto the result of the comparison, and wherein the computer device is setup to calculate a precise amount value (Va) of a structure modifyingagent to be added to the melt in response to the choosen controlcoefficient (γ, φ, ρ_(B) and κ). the computer being set up to send asignal corresponding to said amount value to said means (18), whereby acorrect amount of structure-modifying agent is added to the melt (20).12. An apparatus according to claim 11, characterized in that the secondtemperature sensor (12) is arranged in such a way that the coolingcurves recorded near the wall of the sample vessel wall are recorded ina flow-separated area at the base of an essentially spheroidal samplevessel.